What were Node.js isolates? And why are they now dead?

Problem

In 0.7.0, "Experimenetal Isolates support" [sic] was introduced. I never understood this besides some vague idea that they gave threading-like capabilities but without the problems of threads. And maybe were good for solving Node's debugging/error handling story.

But, nobody ever explained what they were, either in that blog, or in the first few Google results. What are isolates? Why were they introduced to Node?

This morning, a bunch of GitHub issues (2662, 2663, 2665, and probably more) were closed with the comment "isolates is dead". What happened? Why did this supposedly good idea, which from what I could tell was the headline feature for 0.7, die?